20-h rule
The 20H rule has nothing to do with crosstalk. It is supposed to be a method of reducing emission from the edge of a circuit board where short wavelength RF is present. The idea is to pull back the edge of a power plane by 20x the height above the adjacent ground plane to change the field shape at the edge of the board.
There is a lot of controversy about the validity of the theory. You can find several papers on the subject, but one recent one is:
h**p://www.sigrity.com/papers/epep2000/epep_20h.pdf
(replace * with t above)